It has been reported (Schulman and Montague, Annals of the New York Academy of Science, Vol. 92, p. 336 (1961)) that it is necessary to produce negative interfacial tension between the emulsion micelles (internal phase) and the external phase (water) of an aqueous emulsion in order to achieve division of the micelles into micelles of a smaller size.
This negative interfacial tension has been defined in terms of the equation: EQU .gamma..sub.i = .gamma..sub.o/w - .pi. (A)
where
.gamma..sub.i = interfacial surface tension PA1 .gamma..sub.o/w = oil-water interfacial tension in the absence of a surfactant PA1 .pi. = spreading pressure of the surfactant at the oil-water interface PA1 F.sub.1 = HC.sub.ch = cohesive force between the hydrocarbon molecules PA1 F.sub.2 = (S.sub.lp.sup.. HC.sub.ah) = adhesive attraction between the lipophilic end of the surfactant and the molecules of the hydrocarbon PA1 F.sub.3 = LP.sub.ch = cohesive force between the lipophilic ends of the surfactant molecules PA1 F.sub.4 = (S.sub.hp.sup.. W.sub.ah) = adhesive attraction between the hydrophilic ends of the surfactant and the water molecules PA1 F.sub.5 = S.sub.hpch = cohesive force between the hydrophilic ends of the surfactant molecules PA1 F.sub.6 = W.sub.ch = cohesive force between the water molecules
Manifestly, either an increase in .pi. or a decrease in .gamma..sub.o/w will result in a negative value for .gamma..sub.i thus resulting in division of the emulsion micelles into micelles of a smaller size.
The present invention is best understood when equation A is expanded to describe in greater detail the forces which affect the value of .gamma..sub.i. This expanded equation is represented as follows: EQU .gamma..sub.i = (F.sub.4 + F.sub.6 - F.sub.5) - (F.sub.2 + F.sub.1 - F.sub.3) (B)
the forces F.sub.1 through F.sub.6 are represented schematically in FIG. 1 of the drawing wherein an emulsion micelle comprising the internal phase of an emulsion is designated generally by the numeral 10. The emulsion micelle is comprised of molecules of a hydrocarbon 12 which is water insoluble. The hydrocarbon 12 is represented by four small circles enclosed within a large circle and it is to be understood that the smaller circles are merely representative of the presence of the hydrocarbon and are not intended to indicate hydrocarbon molecules. Disposed around the outer periphery of the micelle 10 are a plurality of surfactant molecules 14. Each molecule 14 is comprised of a lipophilic end 16, a hydrophilic end 18, and a connecting chain 20 (FIG. 1). The lipophilic end 16 is soluble in the hydrocarbon 12 and the hydrophilic end 18 is soluble in the external aqueous phase which is represented generally by the numeral 22 and is comprised of a plurality of water molecules 24. A certain number of surfactant molecules 14, which have not yet found their way into the internal phase are also present in the external phase 22. In general, the surfactant molecules 14 may be of the nonionic, cationic or anionic type although it is preferable, as will be explained in greater detail hereinafter, to utilize a nonionic surfactant in combination with a small quantity of an anionic surfactant. Assuming the anionic surfactant is in the form of a sodium salt, the sodium ions will migrate toward the negative charge layer, which is thought to be a static charge, surrounding the micelle 10 and indicated by a broken line in FIGS. 1-3.
Referring again to equation B and to FIG. 1, the forces F.sub.1 through F.sub.6 are defined as follows:
Transferring the above representations for F.sub.1 through F.sub.6 to equation B yields an equation: EQU .gamma..sub.i = [(S.sub.hp.sup.. W.sub.ah) + W.sub.ch - S.sub.hpch ] - [(S.sub.lp.sup.. HC.sub.ah) + HC.sub.ch - S.sub.lpc ] (C)
for the interfacial tension of an aqueous emulsion.
Prior attempts to reduce the micelle size of aqueous emulsions have, in part, been directed to adding large quantities of a nonionic surfactant with a low hydrophilic-lipophilic balance number. This lowers the factor S.sub.hp.sup.. W.sub.ah (F.sub.4) in equation (C) and causes the overall factor of .gamma..sub.o/w (equation B) to be reduced. This causes .gamma..sub.i to assume a negative value and results in further micelle division. Since surfactants of this type are largely water insoluble, dilution of the emulsion is not possible and the technique can be used successfully only where a gel is an acceptable end product. Another disadvantage of utilizing a large excess of such a surfactant is that the surfactant molecules eventually become so crowded around the outside of the micelle that compaction occurs and the factor S.sub.lpch (F.sub.3) dominates all others thereby preventing .gamma..sub.i from assuming a negative value and limiting the extent to which further reduction in micelle size can occur.
Still another known technique for reducing the micelle size of an emulsion is the application of heat. This can be utilized when the forces of compaction prevent further micelle division by the addition of more surfactant. Heat causes the van der Waals' forces to be temporarily broken and the value of S.sub.hp.sup.. W.sub.ah (F.sub.4) in equation (C) to be reduced to give .gamma..sub.i a negative value and the emulsion micelles to undergo further division. This technique cannot be utilized with many compounds, however, because of the danger of decomposition from heating. Thus, most biologically active compounds are exempt from this procedure.
A third known method of reducing the micelle size of an emulsion is to introduce medium chain length fatty acids or alcohols into the emulsion. This tends to break or damp the van der Waals' forces, decreasing the values of S.sub.hpch (F.sub.5) and S.sub.lpch (F.sub.3) in equation (C). This technique is also limited in the extent to which it can reduce the micelle size and is not applicable to dilute aqueous emulsions because of the water insolubility of the fatty acids and alcohols used.
To avoid the disadvantages of the prior art methods discussed above and yet achieve a dilute aqueous emulsion of an unsaturated organic compound characterized by emulsion micelles of a size approaching that of molecules in solution, it has been found that a polyanionic or polycationic water soluble resinous compound can be utilized as a "pumping agent." To achieve micelle size reduction without encountering the limiting factor of compaction as discussed above, the resinous compound is introduced to form a relatively stationary external phase charge situation. The resin thereby achieves a reduction in the interfacial tension without actually becoming a part of the interface between the internal phase (emulsion micelles) and the aqueous external phase.
Referring to FIG. 2 of the drawing, the emulsion micelle 10 is disposed in the aqueous external phase 22 as previously described with a plurality of surfactant molecules 14 surrounding the micelle 10. A quantity of a polyacrylic acid polymer represented by the formula RCO.sub.2 H is disposed in the external phase 22, with each molecule of the polymer being designated by the numeral 26. It is to be understood, of course, that the formula RCO.sub.2 H for the molecules 26 is only representative of the actual formula for the molecules which would have a molecular weight in the range of from 250,000 to 3,000,000 with an enormous number of COOH groups in each molecule. When NaOH is added to the emulsion the following reaction occurs: ##EQU1## with a loose association bond between the Na.sup.+ ions and the negative oxygen of the CO.sub.2.sup.- radical resulting in an overall negative charge on the repeating resin group RCO.sub.2. As NaOH continues to be added more and more of the repeating groups in the polymer chain will become charged while the pH of the emulsion will remain relatively constant because of the OH.sup.- ions from the NaOH combining with the displaced H.sup.+ ions from the resin to form water.
The resulting increased charge in the external phase of the emulsion will cause division of the micelle 10 as a result of the following forces. First, the repulsive charge forces F.sub.7a and F.sub.7b (FIG. 2) between the overall negatively charged resin molecules 26 and the negative charge layer surrounding the micelle 10, and the similar repulsive charge between the Na.sup.+ ions associated with the polymer acid groups and the Na.sup.+ ions which surround the micelle 10, act as a positive charge pressure which is a counter force to the water interfacial tension. Second, because of the attractive force F.sub.8 between the polar water molecules 24 and the charged resin molecules 26, the former will tend to migrate toward the latter, thereby reducing the attraction between the water molecules 24 and any surfactant molecules 14 which are present in the external phase. This in turn frees the molecules 14 to enter the micelle 10 as a result of their lipophilic attraction for the latter. The result is an increase in the value of S.sub.lp.sup.. HC.sub.ah (F.sub.2) (equations B and C above) which contributes to a negative value for .gamma..sub.i. Third, the migration of H.sub.2 O molecules 24 to the charged resin molecules 26 removes the former from the surface of the micelle 10 decreasing the value of S.sub.hp.sup.. W.sub.ah (F.sub.4). Fourth, it is thought that the presence of the charged resin molecules 26 also decreases the factor W.sub.ch (F.sub.6) which further lowers the value of .gamma..sub.i.
With all of the various factors working to reduce .gamma..sub.i, the latter assumes a negative value and the micelles 10 divide into micelles 28 of a smaller size as indicated in FIG. 3. Division will continue until the increased surface area of the micelles 28 causes the surface tension to increase until .gamma..sub.i reaches zero and equilibrium exists.